Spark plug electrode construction



Dec. 15, 1970 c. J. EATON 3,548,239

SPARK PLUG ELECTRODE CONSTRUCTION Filed Sept. 5, 1968 EYE-3- ll. ll "5 INVENTOR.

P: \W EARL JEATUN.

TIE-E- W A TTYE.

3 548 239 SPARK PLUG nrnrnbnn coNsrnUorroN Carl J. Eaton, Toledo, Ohio,assignor to Champion Spark Plug Company, Toledo, Ohio, a corporation ofDelaware Filed Sept. 3, 1968, Ser. No. 756,743 Int. Cl. Hillt 13/20 US.Cl. 313-136 8 Claims ABSTRACT OF THE DISCLOSURE An improved centerelectrode construction for spark plugs used in high performance internalcombustion engines. The body portion of the electrode comprises acorrosion resistant metallic sheath which has a sealed end and which isfilled with a heat conductive metal. A headed, fine wire firing tip of anon-weldable, corrosive nesistant metal is inserted into a cap which isthen welded to the sealed end of the metallic sheath.

BACKGROUND OF THE INVENTION This invention relates broadly to sparkplugs, and, more particularly, to a fine wire tip electrode which can beincorporated into a spark plug assembly for high performance internalcombustion engines.

It is desirable to construct the center electrodes of spark plugs usedin highly corrosive conditions of a material having a high degree ofcorrosion resistance. Spark plugs made of expensive corrosion resistantmaterials generally employ small sized electrodes and are often referredto as fine wire spark plugs. The center electrode of a typical fine wirespark plug terminates at its firing end in a tip constructed of a highlycorrosion resistant material. Platinum and its alloys have been used forthis purpose in the prior art. Such materials can be cold worked, and inprior art electrodes having a platinum or alloy tip, a head or shoulderportion is formed on a platinum rod by cold upsetting.

It has been discovered that other elements, for examples, iridium,tungsten, molybdenum, ruthenium, rhodium, and alloys thereof are oftensuperior to platinum for use in spark plug electrodes which aresubjected to arcing in the corrosive atmosphere of an internalcombustion engine. However, these elements are much more diflic'ult towork than is platinum because of their fibrous characteristics whichrender them very brittle. It is virtually impossible to form heads, onrods made of materials such as iridium, by a cold upsetting operation.

A portion of the center electrode immediately above the tip electrodeoften consists of a conductor such as silver. When platinum tips areused, a chemical surface bond is formed between the platinum tip and thesilver. However, this is not true when, for example, an iridium tip isused. Because no appreciable chemical or metallurgical bonding occurs,the iridium tip has a tendency to move or rotate in its seat.

Attempts have been made to place a spherical head on an iridium rod,which head is then embedded in the silver portion of the centerelectrode. However, when the iridium rod is heated to the melting pointand then molded into the desired shape, recrystallization occurs.Recrystallized fibrous metals are extremely brittle and, as such, arevery susceptible to tensile or vibrational failures. One solution tothis problem is described in US. Pat. No. 3,315,113, which discloses amethod for forming a shoulder on fine pieces of iridium rods.

When using a shouldered tip, the electrode is normally constructed byinserting the tip into acorrosive resistant United States Patent 0sleeve until the shoulder abuts a flange at the end of the sleeve. Asilver core is then cast in the hollow sleeve to hold the tip in placeand to provide good heat conduction along the electrode. However, sinceno chemical bonding occurs between the silver core and the iridium tip,the tip may move or rotate. Such movement may eventually cause sparkplug failure when the tip loosens and falls out of the sleeve. Even whena chemical bond is formed, for example between silver and platinum, thesilver is subject to corrosion and the tip may eventually fall out ofthe sleeve.

SUMMARY OF THE INVENTION According to the present invention, the centerelectrode for a spark plug comprises a body portion having a heatconductive core and a line wire firing tip. The wire tip is attached tothe cored body portion by heading one end of the wire, inserting theheaded end into a cap such that the other end projects through the cap,and then welding the cap to a sealed end of the cored body portion.

The cored body portion of the electrode generally comprises acylindrical sleeve of nickel, nickel alloy, or some other alloy havingsimilar corrosion resistant properties. An axial hole is bored partiallythrough the cylindrical sleeve, leaving a thin wall sealing one end, andthe bore is filled with a copper or silver heat conductive slug.

The tip is constructed from a short piece of iridium wire or some othersuitable corrosion resistant wire. One end of the wire, the firing end,is inserted into a recess in a die. A head is formed on the end of thewire which projects from the die by moving a second die against theprojecting end and simultaneously applying an electric current betweenthe two dies. The current must be sufficient to heat the wire to atemperature at which it can be hot headed, but must be applied for anextremely short period of time so that only a minor amount ofrecrystallization occurs.

A small cap is used to attach the tip to the body portion of theelectrode. The cap must be made of a corrosive resistant material whichis easily welded to the plugged end of the sleeve, preferably of thesame material as the sleeve. The cap is constructed with a small holethrough it which will pass the firing end of the tip but will not passthe headed end of the tip. In assembly, the firing end of the tip isinserted through the hole in the cap such that the headed end of the tiplies within the cap. The cap is then welded to the plugged end of thecored body portion of the electrode and the remainder of the spark plugis assembled.

With the improved electrode construction, the headed end of the finewire tip may be irregular and define small air spaces between the capand the adjacent end of the cored body portion. Corrosive gases from thecombustion chamber may leak into these small air spaces, but the gasescannot attack the easily corroded copper or silver heat conductive coreof the electrode as in prior art electrode constructions.

It is a primary object of the present invention to provide a new andimproved fine wire electrode construction for spark plugs.

Another object of the invention is to provide a spark plug having animproved iridium tip electrode construction.

Still another object of the invention is to provide an improved sparkplug electrode having a heat conductive core and a fine wire tip andconstructed such that the corrosive combustion gases cannot attack theeasily corroded heat conductive core.

Further objects and advantages of the invention will become apparentfrom the following detailed description of the drawings.

3 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional viewof a spark plug assembly embodying the invention;

FIG. 2 is a bottom end view of a spark plug assembly embodying theinvention;

FIG. 3 is an exploded view of a headed fine wire tip and a capconstructed according to the instant invention;

FIG. 4 is a fragmentary sectional view of the bottom of the cored bodyportion of the electrode, the tip and the cap, prior to welding the capto the bottom end of the body portion;

FIG. 5 is a fragmentary sectional view, similar to FIG. 4, showing aportion of an assembled spark plug electrode constructed according tothe instant invention; and

FIG. 6 is a diagrammatic view showing apparatus for heading the finewire electrode tip.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, aspark plug assembly embodying the invention is generally indicated bythe reference numeral 10. The spark plug assembly 10 is of the fine wirevariety and is used in high performance internal combustion engines,such as aircraft engines, where highly corrosive conditions are present.The spark plug assembly 10 generally comprises an annular insulator 11having an axially stepped bore 12 extending therethrough; a centerelectrode assembly 13 fixed in the stepped bore 12; a metallic sleeve 14surrounding the insulator 11; and an outer metallic shell 15. Groundelectrodes 16 extend inwardly at a lowermost end 17 of the metallicshell 15.

The lower portion of the center electrode assembly 13 includes a ceramicsealing material 18 which is in contact with the upper end 19 of anannular metallic sheath 20. The sheath 20 is constructed of a corrosionresistant material, such as nickel or nickel alloy, and has an enlargedflange 21 at its upper end. The flange 21 seats on a shoulder 22 withinthe stepped bore 12 of the insulator 11. The upper end 19 of themetallic sheath 20 has an opening 23 which communicates with a centralopening portion 24 in the metallic sheath 20. An electrode 25 projectsupwardly from the central portion 24 through the opening 23 and throughthe sealing material 18. The lower part of the electrode 25 is inelectrical communication with a core 26 which fills the central portion24 of the metallic sheath 20. The core 26 may consist of silver, copper,or other suitable materials having high thermal conductivity.

The central portion 24 of the metallic sheath 20 does not extendcompletely through the metallic sheath 20: a wall 27 remains, sealing orclosing the lower end of the sheath 20. The corrosion resistant wall 27protects the silver or copper core 26 from exposure to the highlycorrosive gases present in the combustion chamber of an internalcombustion engine. However, the wall 27 should be sufficiently thin suchthat a good heat transfer path is maintained from an attached firing tip28, through the Wall 27, and to the core 26.

The firing tip 28, according to the present invention, comprises a shortpiece of wire coaxially aligned with and attached to the lower end ofthe metallic sheath 20. This attachment is made with a cap 29. The upperpart of the center electrode assembly 13 includes a conductive seal inelectrical communication with the electrode 25, a resistor 31, a spring32, and a terminal stud 33 which threadably engages the insulator 11.

Referring now to FIGS. 3-5, the firing tip 28 and the end cap 29 areshown in detail. The tip 28, which is constructed from a shortcylindically shaped piece of wire, has a shank portion 34, a firing endand an enlarged headed end 36. With this design, the firing tip 28 canbe constructed of an extremely expensive metal, such as 4 iridium, usingonly small quantities of the metal and having negligible material Waste.

The cap 29 comprises a short cylindrically shaped piece of metal havinga stepped axial bore 37. The cap 29 is of the same material as theannular metallic sheath 20, or of any other corrosion resistant metalwhich Will permit the cap 29 to be easily welded to the sheath 20. Thestepped bore 37 includes an enlarged portion 38 adapted to receive theheaded end 36 of the tip 28 and a reduced portion 39 having a diameterjust large enough to receive the shank portion 34 of the tip 28.Construction of the lower portion of the center electrode assembly 13 iscompleted by inserting the firing end 35 of the tip 28 within thestepped bore 37 in the cap 29 such that the headed end 36 of the tip 28lies within the enlarged bore portion 38 and the firing end 35 projectsfrom the cap 29. The cap is welded at 40 to the lower end of the sheath20 over the plug 27. The center electrode 13 is then positioned in theaxially stepped bore 12 which extends through the insulator 11.

When the firing tip 28 is constructed from metal such as iridium,tungsten, molybdenum, ruthenium, rhodium, and alloys thereof,difficulties arise in constructing the headed end 36. Such metals arenonmalleable and cannot be cold worked. If, on the other hand, suchmetals are heated, recrystallization may occur, causing the metals tobecome brittle. The temperature at which one of this group of metalsrecrystallizes varies with the period of time at which the metal remainsat an elevated temperature. The recrystallization temperature decreasesas the time increment increases. Therefore, it is possible to raise thetemperature of, for example, iridium to approximately 2,000? P. withoutsubstantial recrystallization if the increment of elevated temperaturetime is in the order of a fraction of a second, even though iridium,heated over a longer period of time, would normally recrystallize at1,800 F.

It has been found that the fine wire tip 28 can be headed by a processsimilar to that disclosed in US. Pat. No. 3,315,113. Apparatus forpracticing this process is shown in FIG. 6. The apparatus generallycomprises a press 41 having a movable platen 42 and a fixed bed 43. Themovable platen 42 may be moved toward and away from the fixed bed 43 bya connecting rod 44 and a conventional hydraulic cylinder (not shown).An upper die 45 is secured to the movable platen 42 and a lower die 46is secured to the fixed bed 43. The lower die 46 has a cylindricalrecess 47 adapted to receive and hold the shank portion 34 and thefiring end 35 of the tip 28. A step-down transformer 48 supplies currentto the upper and lower dies 45 and 46 by lead-in wires 49 and 50.

An electrical circuit is established between the upper die 45 and thelower die 46 through a tip blank 51. A current is applied to thetransformer 48 to heat the portion of the tip blank 51 which extendsbetween the upper and lower dies 45 and 46. Thereafter the platen 42 ismoved downward to form the headed end 36 on the firing tip 28. The timeincrement during which the current is applied to the transformer 48 issufliciently short that only minor amounts of recrystallization willoccur in the headed end 36. The lower die 46 acts as a heat sink,keeping the shank portion 34 and the firing end 35 of the tip 28 belowthe recrystallization temperature.

Even where recrystallization is not a problem as with platinum andplatinum alloys, the above-described proc ess may be used to form headedends on fine wire spark plug electrode tips.

While the invention has been disclosed in conjunction with a specificform and disposition of the parts, it should be expressly understoodthat modifications and changes may be made without departing from thespirit and scope of the appended claims.

What I claim is:

1. In a spark plug having an insulator with an axial bore therethrough,a center electrode mounted in such bore, said center electrode having atubular metal sheath encompassing a heat conductive metal core adjacentits lower end, an improved center electrode comprising, a metallic capattached to the lower end of said tubular metal sheath, said cap havinga receiving opening and a co-axial shank opening, and a corrosionresistant metal firing tip having a cylindrical shank portion and aheaded end, said headed end of said firing tip being snugly positionedwithin such cap receiving opening, and said cylindrical shank portionextending through such shank opening of said cap and projectingoutwardly from said cap.

2. In a spark plug having an insulator with an axial bore therethrough,a center electrode mounted in such bore, an improved center electrodecomprising, a metallic sheath encompassing a heat conductive metal coremounted adjacent the lower end of said insulator, said metallic sheathhaving a tubular side wall and a continuous integral bottom Wall, acorrosion resistant metal firing tip, said firing tip having a shankportion and a headed end, and means for attaching the headed end of saidtip to said continuous bottom wall of said metallic sheath.

3. An improved spark plug center electrode as claimed in claim 2,wherein said means for attaching the headed end of said tip to saidbottom wall includes a cap, said cap having an axially stepped boretherethrough, the headed end of said tip being located in the largerportion of said stepped bore, the shank portion of said tip beinglocated in the smaller portion of said stepped bore and projecting fromsaid cap, and means for attaching said cap to said sheath.

4. An improved spark plug center electrode as claimed in claim 3,wherein said means for attaching said cap to said sheath is a weldingmaterial.

5. An improved spark plug center electrode as claimed in claim 4,wherein said tip is constructed of a metal selected from the groupconsisting of iridium, platinum, tungsten, molybdenum, ruthenium,rhodium, and alloys thereof.

6. An improved spark plug center electrode as claimed in claim 4,wherein said tip is constructed from iridium.

7. In a spark plug having an insulating core with an axial boretherethrough, a center electrode mounted in such bore, an outer shellsurrounding said insulating core, and at least one ground electrode inelectrical communication vvith said outer shell, an improved centerelectrode comprising: a heat conductive metallic core, a corrosionresistant metallic sheath having an end completely surrounding at leastone end of said core, a corrosion resistant metallic cap welded to saidend of said metallic sheath, said cap forming a chamber with saidsheath, a corrosive resistant firing tip, said firing tip having a shankportion and a headed end, the headed end of said tip being located insaid chamber, the shank portion of said tip projecting through anopening in said cap and into spark gap relationship with such groundelectrode.

8. The improved spark plug center electrode of claim 7 wherein said tipis constructed from iridium.

References Cited UNITED STATES PATENTS 1/1959 Adair 3l3143 X 4/1967Lever 313-136 US. 01. XR, 313 141

